core-data - 具有 20 条最后记录的 NSPredicate

Question

我正在处理核心数据，我尝试使用最后 20 条记录，setFecthLimit但在这些最后的记录中我想获得未读计数，我使用这个

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Post" inManagedObjectContext:_managedObjectContext];
[request setEntity:entity];
[request setFetchLimit:20];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"isRead = NO OR isRead = NIL"];
[request setPredicate:predicate];                    

NSError *error = nil;
NSMutableArray *records = [[_managedObjectContext
                                            executeFetchRequest:request
                                            error:&error] mutableCopy];

但它总是返回我 20，有人可以帮我吗？

Answer 1

一个简单的解决方案是删除谓词。抓取 20 个对象，然后根据谓词过滤它们。

例如：

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Post" inManagedObjectContext:_managedObjectContext];
[request setEntity:entity];
[request setFetchLimit:20];                    

NSError *error = nil;
NSArray *records = [_managedObjectContext executeFetchRequest:request
                                                        error:&error];

// do some error checking here...
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"isRead = NO OR isRead = nil"];
// [NSPredicate predicateWithFormat:@"isRead = %@", [NSNumber numberWithBool:NO]];
NSArray *filteredRecords = [records filteredArrayUsingPredicate:predicate];

Answer 2

我认为您的谓词格式有错字：

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Post"]
request.fetchLimit = 20;
request.predicate = [NSPredicate predicateWithFormat:@"(isRead == NO) OR (isRead == NULL)"];
NSSortDescriptor = [...insert your sort descriptor code here...]
request.sortDEscriptos = @[sd];

记住排序，这样你就可以得到最后的 20 个，而不仅仅是随机的 20 个。